1. Field of the Invention
The invention relates to a process for the manufacture of split-resistant improvement webs which are made from paper with a weight per unit area of .ltoreq.60 g/m.sup.2 which have a hardened synthetic resin on the decorative side and are glued on the surface of wood-based panels to provide a decorative and/or protective surface therefor.
2. Description of the Prior Art
It has long been known as described in German Pat. No. 1,053,303, that paper webs may be impregnated with a solution of a hardenable synthetic resin and, after an optionally interposed drying step, coated with a solution of a hardenable synthetic resin. It is noted, in this regard, that as used herein and as is understood in this art, the term coating or coated means that the particular resin applied does not significantly penetrate into or impregnate the paper.
The products obtained from this process are pressed onto the surface of wood-based panels under conditions appropriate for hardening the resin. For this purpose, paper with a weight per unit area of 60-200 g/m.sup.2 is generally used.
When this type of surface improvement method is used, the processor requires a hotpress, whose pressure is sufficiently high such that at the elevated temperatures of the hardening step, a continuous film is formed of the synthetic resin which is in a molten state and which reproduces the surface structure of the platen. For this purpose, pressures of about 20 kg/cm.sup.2 are required.
It is also well known that improvement webs having such synthetic resin may be hardened, without pressure during their manufacture. These carrier webs, containing hardened synthetic resin are glued by the processor to the surface of wood-based panels and are sometimes optionally provided with an additional, subsequent lacquer application. Pressures of 3-5 kg/cm.sup.2 are adequate for the gluing step.
Synthetic resin containing carrier webs and especially those with the additional lacquer application have the disadvantage that they are very brittle because there are relatively large portions of resin in and on the carrier webs. As a result of the brittleness, tears, eruptions and slits in the web may occur during the manufacture and/or processing of the webs and these defects may lead to breakdowns and losses. Although the carrier webs are manufactured in rolls, because of their fragility and brittleness, they are generally supplied to the press in sheet form for further processing. Further processing of these carrier webs on presses fed from rolls is difficult or impossible because of the fragility, sensitivity and brittleness of the material.
As disclosed in German Auslegeschrift No. 1,287,040, it is known that a crude paper with a specific weight of 1 g/cm.sup.3 and a porosity of not more than 100 ml/minute may be used for impregnating by known procedures with hardenable aminoplast resins or their precondensates, dissolved in organic solvents, for the purpose of manufacturing plastic veneers. The resin portion does not amount to more than 25-30% of the weight of the paper. While these products are more flexible and elastic than the carrier webs, the high density of the paper always makes the use of resins dissolved in organic solvents necessary.
More recently, improvement webs have also become known which have a hardened synthetic resin and are based on thin paper with a weight per unit area of .ltoreq.60 g/m.sup.2. Because of their lesser paper weight, these paper webs are less expensive. After they are printed, such papers are first provided with a coating resin. In this state, they are not brittle and are thus well suited for processing on presses which are fed from a roll.
However, if such coated carrier webs are glued to wood-based panels, it turns out that the split-resistance of the improvement layer is inadequate. If, for example, an adhesive strip is glued to the surface of a wood-based panel improved with such a product, and if this strip is then torn from the surface, the improvement layer splits within the plane of the paper. At times, the resin layer may also strip from the paper support.
On the other hand, if this type of supporting web with a low paper weight is impregnated by conventional procedures and is subsequently coated and optionally hardened before or during the coating, the same problems of brittleness of the improvement web occur as occur with improvement webs whose supporting webs have a higher paper weight.
German Auslegeschrift No. 21 41 703 describes a decorative paper characterized by the fact that the base paper is a highly densified, reinforced, parchment-like cellulose paper, kept free of pigment in the paper mass and having a weight per unit area of much less than 50 g/m.sup.2. A higher resistance to splitting is obtained with such a paper, although at the expense of having to use a highly densified, parchment-like paper which must be free of pigment. The parchmentizing represents an additional expensive processing step in the manufacture of paper. Moreover, since the paper must be pigment-free, the basic color of the decorative paper must be applied in an additional printing process.
A resin-impregnated thin paper has also already been prepared by adding synthetic resin to the fiber slurry during the manufacture of the paper. Such papers either are brittle and difficult to handle or they do not have the necessary resistance to splitting. Also, they frequently are porous so that, as the film manufactured from them is glued onto wood-based panels, glue penetrates through the pores and forms spots on the surface of the improved panels.